Many communications systems use contention mode as a means of allowing access to the communication channels such as radio, LAN etc. In communications systems operating contention mode access schemes much effort has been directed at improving channel access by fast, fair and efficient manners.
Typical channel accessing techniques that exist are based on monitoring of the channel before contention. Some techniques are based on the use of control channels to allocate a particular time slot for a user. In voice communication systems it is known that synchronization signals can be intermittently transmitted on a communications channel in an attempt to keep the communications channel open, thereby avoiding any collision.
One form of contention mode termed global interrogation or "polling" operates on SCADA systems where communications exist over one particular communications medium. The SCADA system comprises a number of remote terminal units (RTUs), which may be fixed or mobile radio units that are polled by a central communications unit. Typically in such a system, each RTU is allocated a different time period during which to sense a free channel before transmitting a reply. The highest priority RTU is allocated the shortest time for reply. Thus, when the channel becomes free, after the central unit has polled the RTUs, it will be the highest priority unit that is allowed to access the channel first. The next priority unit has to wait a longer period to sense a free channel before it attempts to access the channel. In this manner there is an orderly sequence for response from all the RTUs. However, this arrangement has been found to be wasteful of channel time.
Modern communication protocols consist of "session" type transmissions as well as one way messages, i.e. one party of the system is talking to another party and the responding party replies using the same communications medium. The communication continues between the two parties on the chosen communications media until the session is ended. The problem of using session mode in the contention environment is that users on other systems have access to the same media and communications channels but may not use the same contention algorithms. Hence, these "third parties" attempt to, and succeed in, interrupting "on going" sessions.
There is therefore a need for a method of allowing "sessions" between two units on a particular media channel to continue their communication to completion, whilst precluding other units from accessing that channel.